Human immunodeficiency virus-1 (HIV-l), the etiologic agent of AIDS, poses a significant threat to public health in this country, and throughout the world. A more effective therapy for HIV-l infection may require a better understanding of the molecular mechanisms involved in retroviral replication. Viral particle assembly, directed by the gag gene product, is one step in the life cycle of the retrovirus that might offer an opportunity for therapeutic intervention. Unfortunately, very little is understood about this step of the viral life cycle. The purpose of this project is to study the structure of retroviral particles, the molecular mechanisms of the particle assembly process, and the incorporation of glycoproteins during the budding event. The specific research objectives of the proposal are: l) a detailed study of the three dimensional structure of intracellular retroviral particles in order to better define the nature of the intracellular assembly event, as well as a structural comparison of intra- and extracellular particles as a method of defining the morphologic changes taking place during particle-membrane association and virus budding; 2) an in-vitro analysis of retroviral particle assembly and Gag protein interactions using the purified products of disrupted viral particles; 3) localization of the viral assembly process within the cell and characterization of the Gag particle transport process by cryo-electron microscopy and; 4) to determine the domains of the Gag and Env proteins required for specific glycoprotein incorporation into virions, and the requirements for incorporation of heterologous glycoproteins.